TWAYBLADE. 171 



flower and emission of the pollen tubes is probably 

 to retain the pollen, which would otherwise fall out 

 of the flower. In this curious manner, however, they 

 are retained in a proper position until the flower is 

 visited by insects, to which they readily adhere ; and 

 which are necessary to ensure the perfect fertility of 

 the plant. 



Listera ovata (the Twayblade, Fig. 126) has been 

 carefully described by Sprengel, by whom the struc- 

 ture and action of the rostellum was, however, mis- 

 understood, and by Dr. Hooker (Philosophical Trans- 

 actions, 1854), who described the flower with accuracy 

 and minuteness ; but the relations of the flower to 

 insects, and consequently the true functions of the 

 various parts,, were first jjerceived by Darwin. The 

 pollen masses lie immediately above the rostellum; 

 the pollen is friable and would not of itself adhere 

 to insects, but this is effected by a very remarkable 

 contrivance (see Hildebrand, p. 53). The moment 

 the summit of the rostellum is touched, it expels a 

 large drop of viscid fluid, which glues the pollen to 

 the insect or other body. A verj- slight touch, even 

 for instance with a human hair, is sufificient to produce 

 this remarkable phenomenon. 



Neottia nidus avis (the Bird's Nest Orchi.=) agrees 

 in the essential points with Listera, though in the 

 position of the honey, &c., it offers minor differences. 



Cypripedium (the Ladies' Shpper, Fig. 127 and 

 128, C. longifolium), the lower lip has the form of a 

 slipper, whence the name. This genus has two fertile 

 anthers, which are rudimentary in other Orchids, 

 while the one which is present in them is represented 



